Just as an slight aside, D&D 5e has a similar system in the DMG, I'm going to have use it. Now, for this... Gimme a few, I have ideas percolating. Might just do the second.
A Sign of Sorcery
On a small island off the southern coast of the main continent, there's a hedonistic witch/sex cult, whose yearly orgiastic sacrificial rites can often lead to death and/or murder. The players are either captives or slaves on a ship bound for the island, however, the captain bet that if they could kill the 'Harlot', the name of the leader of this cult, and stop whatever foul sorcery this always causes, they win their freedom and passage back to the mainland. And possibly some of their old gear. Maybe.
Other than the unliving master behind the cult, the so-called supernatural power is actually done with powerful hallucinogenics (to steal the drug idea from Rawma, sorry dood, too good not to reuse.) The issue is two fold, first the PC's have no idea that the 'Harlot' (the so-called leader) is actually another victim, the second is that the witch cult is actually quite popular with the locals who rather enjoy the break of their monotonous lives by making a festival of sex, drugs and music around it.
They will have an ally in a cloaked 'wizard' and scholar who can point them to the true cause of the supposed sorcery, but they would have to seek him out first.
Should they defeat the true evil, the Wizard would be a powerful ally to have whenever they adventure near or on the island itself, which has many other weird wonders.